System and method for preventing transmission during message reception

ABSTRACT

A communication controller in a device on a network automatically prevents transmission of data onto the network in response to a transmit request from a CPU of the device if a message is being received or has been received but not read or acknowledged.

BACKGROUND OF THE INVENTION

The present invention relates to a communications controller for use in field instruments and other devices of process control systems. In particular, the present invention is a system and method for low latency data packet reception and processing in a communications controller.

In a typical industrial plant, a distributed control system (DCS) is used to control many of the industrial processes performed at the plant. Typically, the plant has a centralized control room having a computer system with user input/output (I/O), disc I/O, and other peripherals as are known in the computing art. Coupled to the computing system are a controller and a process I/O subsystem.

The process I/O subsystem includes I/O ports which are connected to various field devices throughout the plant. Field devices include various types of analytical equipment, silicon pressure sensors, capacitive pressure sensors, resistive temperature detectors, thermocouples, strain gauges, limit switches, on/off switches, flow transmitters, pressure transmitters, capacitance level switches, weigh scales, transducers, valve positioners, valve controllers, actuators, solenoids, and indicator lights. The term “field device” encompasses these devices, as well as any other device that performs a function in a distributed control system.

Traditionally, analog field devices have been connected to the control room by two-wire twisted pair current loops, with each device connected to the control room by a single two-wire twisted pair. Analog field devices are capable of responding to or transmitting an electrical signal within a specified range. In a typical configuration, it is common to have a voltage differential of approximately 20-25 volts between the two wires of the pair and a current of 4-20 milliamps running through the loop. An analog field device that transmits a signal to the control room modulates the current running through the current loop, with the current proportional to the sensed process variable. On the other hand, an analog field device that performs an action under control of the control room is controlled by the magnitude of the current through the loop, which is modulated by the I/O port of the process I/O system, which in turn is controlled by the controller. Traditional two-wire analog devices having active electronics can also receive up to 40 milliwatts of power from the loop. Analog field devices requiring more power are typically connected to the control room using four wires, with two of the wires delivering power to the device. Such devices are known in the art as four-wire devices and are not power limited, as are two-wire devices.

In contrast, traditional discrete field devices transmit or respond to a binary signal. Typically, discrete field devices operate with a 24-volt signal (either AC or DC), a 110- or 240-volt AC signal, or a 5-volt DC signal. Of course, a discrete device may be designed to operate in accordance with any electrical specification required by a particular control environment. A discrete input field device is simply a switch which either makes or breaks the connection to the control room, while a discrete output field device will take an action based on the presence or absence of a signal from the control room.

Historically, most traditional field devices have had either a single input or a single output that was directly related to the primary function performed by the field device. For example, the only function implemented by a traditional analog resistive temperature sensor is to transmit a temperature by modulating the current flowing through the two-wire twisted pair, while the only function implemented by a traditional analog valve positioner is to position a valve between an open and closed position, inclusive, based on the magnitude of the current flowing through the two-wire twisted pair.

More recently, hybrid systems that superimpose digital data on the current loop have been used in distributed control systems. One hybrid system is known in the control art as the Highway Addressable Remote Transducer (HART) and is similar to the Bell 202 modem specification. The HART system uses the magnitude of the current in the current loop to sense a process variable (as in the traditional system), but also superimposes a digital carrier signal upon the current loop signal. The carrier signal is relatively slow, and can provide updates of a secondary process variable at a rate of approximately 2-3 updates per second. Generally, the digital carrier signal is used to send secondary and diagnostic information and is not used to realize the primary control function of the field device. Examples of information provided over the carrier signal include secondary process variables, diagnostic information (including sensor diagnostics, device diagnostics, wiring diagnostics, and process diagnostics), operating temperatures, temperature of the sensor, calibration information, device ID numbers, materials of construction, configuration or programming information, etc. Accordingly, a single hybrid field device may have a variety of input and output variables and may implement a variety of functions.

HART is an industry standard nonproprietary system. However, it is relatively slow. Other companies in the industry have developed proprietary digital transmission schemes which are faster, but these schemes are generally not used by or available to competitors.

More recently, a newer control protocol has been defined by the Instrument Society of America (ISA). The new protocol is generally referred to as Fieldbus. Fieldbus is a multi-drop serial digital two-way communications protocol intended for connecting field instruments and other process devices such as monitoring and simulation units in distributed control systems. Fieldbus allows enhanced digital communication over previous process control loop methods while maintaining the ability to power process devices coupled to the Fieldbus loop and while meeting intrinsic safety requirements.

Two reasonably standardized industrial Fieldbus protocols are Foundation Fieldbus and Profibus. The physical layer of the Fieldbus protocols are defined by Instrument Society of America standard ISA-S50.02-1992, and its draft two extension dated 1995. The Fieldbus protocol defines two subprotocols. An H1 Fieldbus network transmits data at a rate up to 31.25 kilobits per second (Kbps) and provides power to field devices coupled to the network. The H1 physical layer subprotocol is defined in Clause 11 of the ISA standard, part two approved in September 1992. An H2 Fieldbus network transmits data at a rate up to 2.5 megabits per second (Mbps), does not provide power to field devices connected to the network, and is provided with redundant transmission media.

Fieldbus provides significant capabilities for digitally communicating immense amounts of process data. Thus, there is a continuing need to develop process control devices capable of maximizing fieldbus communication efficiency.

BRIEF SUMMARY OF THE INVENTION

A communication controller of a device on a network automatically controls transmission of data so that transmission does not occur while a message is being received. As messages are received, the communication controller stores received message objects which include attributes indicating whether the object has been read. If any received message object indicates an unread message, the communication controller will not execute a transmit request.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a process control system with digital communication between devices over a communication medium segment.

FIG. 2 shows a message format for communications between devices of the process control system of FIG. 1.

FIG. 3 is a block diagram of a device of the process control system.

FIG. 4 is a functional block diagram of a communication controller of the device of FIG. 3.

DETAILED DESCRIPTION Process Control System Overview

The present invention relates to a communication controller for use in field instruments and other devices of process control systems. The purpose of the communication controller is to perform a substantial portion of the link layer processing of messages and timer management, thereby freeing the application processor or CPU to perform other functions. For the purpose of this detailed description, the communication controller will be described in the context of a system using the Foundation Fieldbus communications protocol, although it has general applicability to packet-based communication protocols.

The fieldbus physical layer defines the electrical characteristics of the physical means of transmission and reception of the communications protocol data in the form of a Physical Layer Protocol Data Unit (PhPDU). In addition, the fieldbus physical layer specifies the symbol encoding, message framing, and error detection method. The ISA fieldbus standard defines three signaling speeds and two modes of coupling. For purposes of this description, the invention will be described in the context of the H1 physical layer defined in clause 11 of ISA S50.02 Standard, Part 2. That clause covers a 31.25 Kbps, voltage mode, wire medium, with a low-power option. This option allows for a device connected to the communications medium to receive its operational power from the communications medium. The physical layer can be capable of meeting the intrinsic safety requirements for hazardous environments. The protocol operates on low-grade twisted pair cable and supports multiple devices, in accordance with the voltage and current limitations which are defined by the standard.

FIG. 1 shows a typical process control system 10 including segment 12, power supply 14 and five devices: Link Active Scheduler (LAS) device 20, Link Master (LM) device 22, and basic devices 24, 26, and 28. Segment 12 can support up to thirty-two devices on a single pair of wires. Typically, segment 12 will have from four to sixteen devices, based on loop execution speed, power, and intrinsic safety requirements.

LAS device 20 maintains a central schedule for all the communication between devices on segment 12. LAS device 20 improves the overall communication reliability by sending Compel Data (CD) Data Link Protocol Data Units (DLPDUs) to each device to transmit back cyclic data which is then scheduled to do so. LAS device 20 serves as the local source of Data Link time (DL-time) on segment 12. A DLPDU is the data content of the PhPDU message that is communicated across segment 12.

LM device 22 is configured to take over the responsibilities of LAS device 20 should LAS device 20 fail or become inoperable. Although only LM device 22 is shown in FIG. 1, more than one Link Master device can be present on a segment. This allows for the case if both the Link Active Scheduler and the first Link Master were to fail, then the second Link Master can take over for the Link Active Scheduler. Once the Link Active Scheduler is disabled, the Link Master takes over the functionality of the Link Active Scheduler.

Each device has a unique address called the V(TN), which represents the local node-ID(This Node). In the example shown in FIG. 1, LAS device 20 has address V(TN)=20; LM device 22 has address V(TN)=22; basic device 24 has address V(TN)=A5; basic device 26 has address V(TN)=F3; and basic device 28 has address V(TN)=F5.

LAS device 20 sends Pass Token (PT) and Probe Node (PN) messages to all devices on segment 12. Each of the other devices (LAS device 22 and basic devices 24, 26, 28) send Return Token (RT) and Probe Response (PR) messages back to LAS device 20, as appropriate.

Each basic device 24, 26, 28 only needs to see its own PT and PN messages that are sent by LAS device 20. PT and PN messages have a designation address (DA) encoded in the second byte of the DLPDU. LAS device 20 passes a token (PT) or probes a node (PN) one at a time to all devices on segment 12.

Once basic device 24, 26, or 28 receives a PT message with a designation address equal to that device's unique address (DA=V(TN)), it then will respond back to LAS device 20 with an RT message. If basic device 24, 26, or 28 receives a PN DLPDU with DA=V(TN), it is required to respond back with a PR message.

The transmission of PT and PN messages from LAS 20 and RT and PR messages to LAS 20 creates several messages on segment 12 that a particular basic device 24, 26, 28 does not need to receive and take action on. Each basic device 24, 26, 28 only needs to respond to PT and PN messages addressed to that particular device. Constantly getting interrupted by PT and PN messages from LAS 20 that are addressed to other devices, as well as RT and PR messages from other devices addressed to LAS device 20, can create undue processing time to handle these “nuisance interrupts.” With basic devices 24, 26, and 28, DLPDU filtering can be used to reduce the number of interrupts that the basic device has to process. On the other hand, LAS device 20 must process every message on segment 12.

All devices on segment 12 transmit data onto segment 12 as a Manchester encoded baseband signal. With Manchester encoding, “0” and “1” are represented by transitions that occur from low-to-high and high-to-low, respectively, in the middle of the bit period. For fieldbus, the nominal bit time is 32 microseconds (μsec), with the transition occurring at 16 μsec. The Manchester encoding rules have been extended to include two additional symbols, non-data plus (N+) and non-data minus (N−), wherein no transition occurs during the bit period and the Manchester encoded baseband signal remains high (N+) or low (N−).

Message Format

FIG. 2 shows the format of a Physical Layer Protocol Data Unit (PhPDU) used to transmit messages over segment 12. The PhPDU includes a preamble, a Start Delimiter (SD) a Data Link Protocol Data Unit (DLPDU) and an End Delimiter (ED). The preamble is the first several bits of the PhPDU message. The fieldbus specification allows for one to eight bytes of preamble. The device receiving the message uses the preamble to synchronize with the incoming message. As shown in FIG. 2, the sequence of the first byte of the preamble is 1 0 1 0 1 0 1 0.

The Start Delimiter (SD) immediately follows the preamble. There is one SD per message. The fieldbus specification requires that the SD have non-character data (N+ and N−), which always appear in the SD message in complementary pairs. This encoding scheme makes the SD unique and impossible to confuse with the data portion (DLPDU) of the message. The sequence shown in FIG. 2 for the SD is 1 N+N−1 0 N−N+0.

The DLPDU is a variable length message. It contains a Frame Control (FC) byte as its first byte and a Frame Check Sequence (FCS) check sum as its final two bytes. The length of DLPDU is variable, with a minimum of three bytes (in the case of an RT message) up to a jabber limit of, for example, about 300 bytes.

The End Delimiter (ED) follows the DLPDU. It represents the last byte of any PhPDU message transmitted over segment 12. Similar to the SD, the ED includes non-character data in complementary pairs. This encoding scheme makes the ED unique and impossible to confuse with the DLPDU. The sequence shown in FIG. 2 for the End Delimiter is 1 N+N−N+N−1 0 1.

FIG. 2 also shows a Carrier Detect signal. The purpose of the Carrier Detect signal is to indicate when (a) an incoming PhPDU message is present on segment 12 or (b) a device is transmitting a message onto segment 12.

Start of Transmit (SOT) occurs at the moment that a Transmit Enable (TxE) goes active, i.e., when the preamble of a PhPDU message is first presented to segment 12.

Start of Activity (SOA) occurs after the Carrier Detect signal goes active and has been stable for at least one bit time or two bit times (approximately 16 to 32 μsec). This time depends on when the Carrier Detect goes active with respect to the internal clock of the device receiving the message. This allows the communication controller of the device to ignore noise glitches that are most apt to occur at the front end of the preamble. Additional time is used to synchronize with the bit boundaries to eliminate the potential for short noise bursts on segment 12 being misinterpreted as activity. For a transmitted message, SOA occurs once the Transmit Enable goes active (i.e., the preamble of the PhPDU is presented to segment 12).

Start of Message (SOM) occurs at the beginning of the first bit of when the FC byte is detected for a received message.

SOM_xmt is the Start of Message Transmit, which occurs at the beginning of the first bit of when the FC byte is detected for a transmitted message.

SOMf is an SOM of a received filtered DLPDU. This occurs when the communication controller within the device has detected enough information to make the determination that the incoming message is to be filtered.

End of Message (EOM) occurs at the end of the last bit of the ED being encountered in a received message. End of Transmission (EOT) occurs at the end of the last bit of the ED a transmitted message.

End of Activity (EOA) occurs when the Carrier Detect has gone inactive. The EOA occurs for both transmitted and received DLPDUs.

Device Architecture

FIG. 3 shows a block diagram of the communications portion of basic device 24, which is representative of the architecture in each of devices 20-28. Basic device 24 includes central processing unit (CPU) 30, random access memory (RAM) 32, flash memory 34, communications controller 36, and medium attachment unit (MAU) 38.

In the embodiment shown in FIG. 3, CPU 30 is a microprocessor such as Motorola 68LC302, Motorola Mcore 2075, Motorola PowerPC 850, Atmel Thumb processor AT91M40800 and others. CPU 30 is an 8-bit or higher processor.

In the embodiment shown in FIG. 3, communication controller 36 is an application specific integrated circuit (ASIC) chip that serves as an interface between MAU 38 and CPU 30. It transmits and receives encoded Manchester data to and from external analog circuitry connected to segment 12. After receiving the serial data from MAU 38, communication controller 36 decodes the data, forms the data into bytes, strips off the preamble, SD, and ED, (and, optionally, the FCS bytes) and provides the message data for the link layer to read. For data transmission, communication controller 36 receives bytes of DLPDU data from the link layer and adds the preamble, the SD, optionally generates the FCS, and adds the ED. Communication controller 36 then forms serially encoded Manchester data, which is sent to MAU 38 for transmission on segment 12.

Communication between communication controller 36 and MAU 38 is provided through four signals: RxS, RxA, TxS, and TxE. RxS is the received Manchester Encoded serial data. RxA is the Carrier Detect signal for received data. TxS is the transmitted encoded serial data. TxE is the transmit enable signal.

In other embodiments of the invention, communication controller 36 can be formed on a common integrated circuit with CPU 30. In addition, RAM 32 and flash memory 34 may be combined with CPU 30 in some embodiments. In the case of LAS device 20, CPU 30, RAM 32 and flash memory 34 may be a part of a host computer system of process control system 10.

MAU 38 provides the network connection to segment 12. MAU 38 may be an integrated circuit, or discrete components can be used to form MAU 38.

Communication Controller 36

FIG. 4 is a functional block diagram of communication controller 36. In this embodiment, communication controller 36 includes debounce circuit 42, digital phase lock loop (PLL) 44, front end state machine 46, receive message filtering 48, receive first-in-first-out (FIFO) memory 50, transmit state machine 52, transmit FIFO memory 54, receive/transmit event manager 58, registers 60, clock generation circuitry 62, oscillator 64, timers 68, and CPU interface circuitry 70.

When an incoming message is detected by MAU 38, a Carrier Detect signal is provided to communication controller 36 at the RxA input, and the incoming asynchronized Manchester data is provided at the RxS input. The RxA and RxS inputs are presented to front end state machine 46. Digital PLL 44 recovers and regenerates the clock from the incoming serial Manchester encoded data. This regenerated clock is then used to clock front end state machine 46.

Front end state machine 46 detects the incoming serial bit stream RxS. It strips off the preamble, SD, and ED, and stores the DLPDU into receive FIFO memory 50. Front end state machine 46, together with receive message filtering 48, can be configured to filter out specific frame controls, plus Probe Node (PN) and Pass Token (PT) messages addressed to other devices. Front end state machine 46 keeps track of the number of bytes that have been written into receive FIFO memory 50. The FCS is automatically verified at the end of each message, and can be optionally stored into receive FIFO memory 50.

Front end state machine 46 also provides signals representing specific events it has detected. These include the SOM, SOMf, EOM, SOA, and EOA event pulses.

Front end state machine 46 is activated when the RxA line goes active. Front end state machine 46 then synchronizes with the edges of the preamble field and decodes the Manchester encoded data of the RxS signal. The SOA event indicates that front end state machine 46 has started.

Once the preamble has been detected, front end state machine 46 waits for the Start Delimiter (SD) sequence. After the SD has been detected, front end state machine 46 converts the serial data stream into octets, and writes them to receive FIFO memory 50 in 8-bit bytes. Front end state machine 46 continues writing new octets of data into receive FIFO memory 50 until the End Delimiter (ED) is detected, or until receive FIFO memory 50 is full.

When the ED has been detected, front end state machine 46 waits for the RxA line to go inactive, which is indicated by the EOA event.

With the RxA line inactive, front end state machine 46 returns to its initial state. It remains in that initial state until the next activity on fieldbus segment 12 (i.e., until a Carrier Detect signal is provided at RxA again).

Filtering circuitry is used by basic devices to reduce IRQ loading on messages that are not important to the device. In contrast, a device configured as an LAS must receive all messages on the segment and therefore must have filtering disabled. When filtering is disabled, all received messages will be stored in receive FIFO memory 50 and will be passed on to registers 60 and then to CPU 30. SOMf is a Start Of Message signal for a received filtered DLPDU. It occurs when front end state machine 46 has determined that the received message has detected enough information to determine that the incoming message is to be filtered.

With filtering enabled, messages that are filtered are not stored in received FIFO memory 50. For filtered messages, SOMf will not be generated, therefore no event or IRQ will occur.

Examples of filtered messages are Return Token (RT), idle, Request Interval (RI) and Probe Response (PR) DLPDU messages. These are identified based upon the Frame Control (FC) byte and will always be rejected with filtering enabled. Pass Token (PT) and Probe Node (PN) messages will be accepted if the destination address in the message matches the address for the device. If the destination address does not match, then the PT and PN messages are rejected.

The ability to filter message types based on the FC byte and based upon the destination address reduces the software interrupt loading by limiting the number of interrupt requests (IRQs) that CPU 30 must process.

Front end state machine 46 and receive FIFO memory 50 are used to parse the serial data frames from MAU 38. CPU 30 reads the data from receive FIFO memory 50 and places it in its local memory space to decode the received DLPDU.

Receive FIFO memory 50 is 63 bytes by eight bits wide. Receive FIFO memory 50 will store all of the DLPDU bytes up to three complete received messages (up to a total of 63 bytes). Front end state machine 46 decodes the serial data stream from the filtered RxS signal, and converts it to an 8-bit parallel formatted byte. After the formation of the byte, front end state machine 46 creates a write pulse that stores the coded data into the location that is pointed to by a write pointer. After the write operation is complete, the write pointer is incremented to store the next DLPDU byte.

CPU 30 interfaces with a read pointer to receive FIFO memory 50. Any read from the receive FIFO register of registers 60 (which contains the actual DLPDU data) places the 8-bit data from receive FIFO memory 50 immediately onto the data bus for reading by CPU 30. After the read operation is complete, the read pointer is incremented. This can be continued until receive FIFO memory 50 is empty.

To prevent an overflow condition from occurring in receive FIFO memory 50, there is a register within registers 60 that allows an IRQ to be generated if receive FIFO memory 50 is approaching a full condition. The threshold for generating the IRQ is configurable.

Transmit state machine 52 reads the DLPDU data to be transmitted from transmit FIFO memory 54. The preamble, SD, and ED are automatically inserted. To start transmit state machine 52, the interPDU trigger or, optionally, the Next Scheduled Event trigger needs to be activated to commence the transmit operation. Transmit state machine 52 keeps track of the number of bytes that have been transmitted. An error status will be indicated if there is an underflow or transmitted count violation. The FCS can be optionally transmitted automatically as the last two bytes of the DLPDU.

Transmit state machine 52 encodes the Manchester serial data supplied through interface circuitry 70 on the TxS line to MAU 38 to be presented on segment 12. Transmit state machine 52 also asserts the Transmit Enable (TxE) line at the instant that the first bit the first preamble is sent until the last bit of the ED occurs. Transmit state machine 52 also generates the Start Of Transmission (SOT) event signal when it asserts the TxE line, and generates the End Of Transmission (EOT) event signal when the TxE line returns to inactive.

Transmit FIFO memory 54 stores all of the DLPDU bytes that are required for a message to be transmitted, up to a total of 63 bytes. A configurable threshold can be set to send an IRQ telling CPU 30 when transmit FIFO memory 54 is almost empty. In that way, if more than 63 bytes are required to be transmitted, CPU 30 is notified so that it can add more data to transmit FIFO memory 54. This continues until all DLPDU bytes have been written. CPU 30 writes to transmit FIFO memory 54 using a write pointer, while transmit state machine 52 reads bytes from transmit FIFO memory 54 using a read pointer.

Communication controller 36 works on events, and must be capable of handling the occurrence of multiple events. Examples of events include an SOM, EOM, or EOA for a received message or an EOT for a transmitted message. Receive/transmit event manager 58 manages all of the events that occur for up to a total of three received messages and one transmitted message.

As shown in FIG. 4, receive/transmit manager 58 includes three received message objects labeled rcvmsg1, rcvmsg2, and rcvmsg3, and one transmit message object labeled xmtmsg. In addition, receive/transmit manager 58 includes message queue manager (MsgQmngr) 80, event manager (EventMngr) 82, transmit manager (xmtmngr) 84, and event MUX 86.

Receive FIFO memory 50 is capable of storing the DLPDU bytes for up to three complete received messages. Each of those three messages has a corresponding object rcvmsg1, rcvmsg2, and rcvmsg3. Each object contains the status of all of the IRQs, message errors, and time stamping that occur for its corresponding received message. This information constitutes the event data for that message.

The status of all IRQs, message errors, and time stamping that occur for a transmit message are stored in the xmtmsg object. The stored information constitutes the event data for the transmitted message.

MsgQmngr 80 controls the selection and the enabling of the three received message objects. Only one received message object can be active at a time. MsgQmngr 80 allows the events to be associated with the active received message object. In the case of a fourth message being received before the other three messages have been acknowledged by CPU 30, MsgQmngr 80 disables any further messages from being received until the event data has been read or acknowledged.

EventMngr 82 manages the order of occurrence of events. As events occur, event manager 82 assigns each event an order of occurrence identification (OOO_ID). This allows CPU 30 to read the events one at a time as they occur. CPU 30 must acknowledge each event as it occurs. After the first event has been acknowledged, the subsequent event will be ready for CPU 30 to read.

Xmtmngr 84 monitors the InterPDU trigger (InterPDU_trig) and the Next Scheduled Event trigger and initiates the Transmit Trigger Command (Xmt_Trig_Cmd) to transmit state machine 52 to cause the next message to begin to be transmitted.

Communication controller 36 includes registers 60. These registers, designated Reg00-Reg3F, can be written to and read from by CPU 30. Interrupts (IRQs) are also handled through registers 60.

Clock generation circuitry 62 receives an external clock and either uses that clock or the clock signals from its internal oscillator 64 to generate all necessary clock signals for communication controller 36.

Clock generation circuitry 62 preferably has the capability of currently adjusting both its node timer and its octet timer clock rates. This allows communication controller 36 to synchronize the relationship of its Node Time with the Link Address Scheduler (LAS 20). Octet Time is used for internal message timing, while Node Time is used to share a common sense of time across segment 12.

Timer 68 will be divided into two groups, representing different senses of time. A first set of timers, called segment timers, operates based on a variable clock rate produced by clock generation circuitry 62 under software control from CPU 30. A second set of timers, called message timers, operates on a fixed rate clock.

There are two segment timers in communication controller 36. The first segment timer is a Node Timer, which has a clock tick rate of 31.25 μsec (32 kHz). The Node Timer is used to implement the Next Function Block Execution Time, Link Schedule Time V(LST), and Data Link Time (DL-Time).

The second segment timer is the Octet Timer, which has a clock tick rate of 2 μsec (500 kHz). The Octet Timer is used for the Next Scheduled Event trigger (which interfaces to transmit state machine 52 for transmitting messages at a specific time). When the clock rate is adjusted, the Node and Octet timers will track one another at the same rate. This is because the clock signals driving the Node Timer and the Octet Timer are derived from a common variable clock.

The message timers are started and stopped based upon fieldbus message events (transmit and receive). The message timers include an inactivity timer, an interPDU delay timer, a receive reply timer, a transmit reply timer, a delegated token recovery timer.

The inactivity timer is a decrementing counter. It is used for measuring the idle time between two PhPDUs. The inactivity timer works on both filtered and non-filtered received messages as well as any transmitted messages on segment 12. When commanded to start, the inactivity timer will decrement every 16 μsec. The inactivity timer starting point is determined from a configurable set point preloaded into one of registers 60. The decrementing of the inactivity timer can be cancelled or stopped via events that are related to either a received or transmitted message. If the timer ever reaches 0 or expires, an IRQ will be generated. The inactivity timer will remain at 0 until the IRQ is acknowledged. If the IRQ remains high, no additional message events that occur will effect the inactivity timer until this IRQ is acknowledged.

The interPDU delay timer is an incrementing counter. It is used in conjunction with a V(MID) threshold register to implement the fieldbus V(MID) minimum-interPDU delay that insures a minimum time duration (or gap time) of non-transmission between a transmitted or received message. The interPDU timer is affected by both filtered and non-filtered received messages as well as any transmitted messages on the segment 12. When there is no fieldbus activity, the interPDU timer will continuously increment. Once the count value equals or exceeds a predetermined value stored in one of registers 60, the InterPDU trigger signal will go active. This signal is used for determining that the interPDU delay time has been met. This signal interfaces to xmtmngr 84 to give the command that a transmitted DLPDU can commence.

The receive reply timer is a decrementing counter. It is used to allow a subscribing device to monitor for an immediate response to a Compel Data (CD) DLPDU. It is also used for a device to monitor its own address when coming online. When commanded to start, the receive reply timer will decrement every 16 μsec. The receive reply timer starting point is determined from a configurable 16-bit set point preloaded into one of registers 60. The decrementing of the receive reply timer can be cancelled or stopped via either a SOM or SOT event. If the receive reply timer ever reaches 0 or expires, an IRQ will be generated. The receive reply timer will remain at 0 until the IRQ is acknowledged. If the IRQ remains high, no additional message events that occur will affect the receive reply timer until this IRQ is acknowledged.

The transmit reply timer is a decrementing counter. It allows a device to monitor for an immediate response after transmitting one of several DLPDUs (e.g., compel data, pass token). When commanded to start, the transmit reply timer will decrement every 16 μsec. The transmit reply timer starting point is determined from a configurable set point preloaded into one of registers 60. The decrementing of the transmit reply timer can be cancelled or stopped via either a SOM event or SOT event of any transmitted DLPDU except that of a Probe Node (PN). If the transmit reply timer ever reaches 0 or expires, an IRQ will be generated. The transmit reply timer will remain at 0 until the IRQ is acknowledged. If the IRQ remains high, no additional message events that occur will affect the transmit reply timer until this IRQ is acknowledged.

The delegated token recovery timer is a decrementing counter. It is used for monitoring the idle time of receiving a delegated token from another device. The delegated token recovery timer works on both filtered and non-filtered received messages as well as any transmitted messages on segment 12. When commanded to start, the delegated token recovery timer will decrement every 16 μsec. The delegated token recovery timer starting point is determined from a configurable set point preloaded into one of registers 60. The decrementing of the delegated token recovery timer can be cancelled or stopped via events that are related to either a received or transmitted message. If the delegated token recovery timer ever reaches 0 or expires, an IRQ will be generated. The delegated token recovery timer will remain at 0 until this IRQ is acknowledged. If the IRQ remains high, no additional message events that may happen to occur will affect the delegated token recovery timer until this IRQ is acknowledged.

Preventing Message Transmission During Message Reception

The Fieldbus data link layer protocol specification, ANSI/ISA-S50.02-1997 Part 4, section 6.2.6 states that a device holding the dominant token (i.e., the device that will transmit next) on the Fieldbus network must drop that token, and therefore cancel any pending transmission, if it receives new data while waiting to begin transmitting.

The dominant token holder waits to begin transmission on segment 12 for two reasons. First, the required interval of minimum DLPDU delay, V(MID), has not yet passed. Second, the software ruiming on CPU 30 is busy preparing the next DLPDU to send.

Communication controller 36 supports both reception and transmission of DLPDUs. Communication controller 36 prevents the transmission of a DLPDU if a new DLPDU is received prior to the start of transmission, even if the software in CPU 30 is as yet unaware of the new DLPDU reception. Communication controller 36 notifies the software that the transmission was prevented.

The present invention fills a “timing window” that exists from the time a new DLPDU is received by communication controller 36 until the time that the software in CPU 30 becomes aware that the DLPDU was received. During this timing window, any attempt by the software to start transmission will be prevented by communication controller 36.

The automatic prevention of transmission is achieved by adding a Boolean attribute “Unread” to each object (rcvmsg1, rcvmsg2, and revmsg3) in receive/transmit event manager 58 that represents a received DLPDU. The “Unread” attribute is not directly accessible by software. In other words, it cannot be written to or read from received message objects rcvmsg1, rcvmsg2, or rcvmsg3 by CPU 30.

Received message object rcvmsg1 contains all data (IRQ's, status, and error messages) associated with a first received message. Similarly, rcvmsg2 contains all event data associated with a second received message, and rcvmsg3 contains all event data associated with a third received message.

Upon the beginning of reception of a DLPDU, the “Unread” attribute in the associated received message object is set to TRUE. When software in CPU 30 accesses any readable attribute of the received message object, the “Unread” attribute in that object is set to FALSE. If the received message object is acknowledged, either externally by the software in CPU 30 or internally by communication controller 36, the “Unread” attribute in that received message object is set to FALSE.

An additional Boolean attribute called “Receive Pending” is produced by the received message object. “Receive Pending” is TRUE if any of the “Unread” attributes of the three received message objects is TRUE. In other words, the “Receive Pending” attribute represents the logic OR of the three individual “Unread” attributes.

If software in CPU 30 makes a request to begin transmission, the ability to transmit will depend upon the status of the “Receive Pending” attribute. If “Receive Pending” is FALSE, communication controller 36 allows transmission to begin. The software is notified that the transmit request was successful, and later the software is notified that the transmission is complete.

If the “Receive Pending” attribute is TRUE, communication controller 36 does not allow transmission to begin. Instead, communication controller 36 notifies the software that the transmit request failed.

After each transmit request, the software checks registers 60 to see whether the request was successful. If the request was successful, the software continues to hold the dominant token and waits for transmission to complete. If the transmit request was unsuccessful, the software immediately drops the dominant token.

The present invention eliminates the software timing window which exists between the time that a new DLPDU is first received and the time when the software becomes aware that the new DLPDU has been (or is being) received. Either transmission starts without a receive DLPDU waiting for the software to acknowledge it, or transmission is denied and communication controller 36 later reports a received DLPDU to CPU 30.

The present invention makes it possible for software to successfully request a response DLPDU transmission prior to the end of activity (EOA) of a received immediate response request DLPDU to improve Fieldbus network performance. Since the received immediate response request DLPDU has already been accessed by software, “Receive Pending” will be FALSE at the moment the response DLPDU transmission request is made.

Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. 

1. A circuit for preventing message transmission onto a network while a message is being received from the network, the system comprising: a receiver circuit for receiving messages from a network; a receive message buffer for storing received message objects each received message object having an associated unread status attribute indicating either TRUE or FALSE, circuit having a receive pending attribute that is set to TRUE if the unread state attribute of any received message object stored in the receive message buffer is set to TRUE; a transmit message buffer for storing data to be transmitted; a transmitter that transmits the data in the transmit message buffer on response to a transmit request only if the receive pending attribute is set to FALSE.
 2. A method of controlling message transmission onto a network, the method comprising: receiving a transmission request from software; storing data to be transmitted; receiving messages from the network; storing message objects based upon messages received in a receive buffer; producing a receive pending attribute associated with a received message objects, the receive pending attribute being set to TRUE automatically upon receipt of a message object, the receive pending attribute indicating that any message object stored in a receive buffer has not been read; testing status of the receive pending attribute; and processing the transmission request depending on status of the receive pending attribute.
 3. The method of claim 2, wherein processing the transmission request comprises: transmitting the data stored if the receive pending attribute is FALSE; and notifying the software that the transmission request was successful.
 4. The method of claim 3 and further comprising: notifying the software when the transmission of the data is complete.
 5. The method of claim 2, wherein processing the transmission request comprises: notifying the software that the transmission request failed if the receive pending parameter is TRUE.
 6. A device for communicating over a communication medium, the device comprising: a medium attachment unit (MAU) for receiving and transmitting messages on the communication medium; a central processing unit (CPU) for processing data contained in messages received and creating data to be contained in messages to be transmitted; and a communication controller for interfacing between the MAU and the CPU, the communication controller maintaining received message objects for messages received, and controlling transmission of data in response to a transmit request from the CPU if a receive pending attribute associated with the received message objects indicates that there are no unread received messages. 